The invention relates to a method for purifying waste water by means of anaerobic treatment (digestion) in an anaerobic treatment zone. The method is preferably applied to the treatment of highly concentrated problematic waste water, e.g., seepage water from a refuse dump, that contains organic impurities (COD, chemical oxygen demand) dissolved in high concentrations, as well as AOX (absorbable organic halogen compounds), and is therefore not suitable for discharge into a conventional communal purification plant with aerobic treatment zones. Problematic waste water of this kind can be subjected to an effective and economical elimination of COD and AOX in an anaerobic treatment zone (digestion reactor or digestion tower).
The invention also relates to a purification plant for carrying out said method.
Preferably, the invention can be applied for the simultaneously, separate treatment of communal waste water and highly concentrated problematic waste water, particularly refuse dump seepage water. Conventional, communal waste water from households, commerce or industry is purified in communal purification plants in which the degradation of the dissolved pollutants takes place in an aerobic biological treatment zone. In particular, it can consist of an activated sludge tank and/or aerobic biological reactors that are operated with high or low sludge loading. In modern plants, elimination of nitrogen by means of nitrification and denitrification takes place in addition to the degradation of the organic carbon compounds.
Known from EP 0 354 906 and WO 99/23038 are methods for aerobic waste water purification, in which returned sludge and returned water, which come from a secondary sedimentation tank that is downstream from the activated sludge tank and both of which are oxygen-rich and contain nitrate, are sent to a region, e.g., a primary sedimentation tank or its admission section, which is upstream of the activated sludge tank and has waste water flowing through it, in order to initiate nitrification/denitrification and/or a biological phosphorus elimination there.
On the other hand, there is also problematic waste water with a concentration of dissolved impurities that is so high that they cannot be effectively treated in the communal purification plants discussed above. An example of this kind of problematic waste water is seepage water from a refuse dump. It is known that highly concentrated problematic waste water can be subjected to anaerobic treatment in a digestion reactor. By doing this, organic impurities (COD), even those present in high concentrations, can be anaerobically biologically degraded, resulting in the creation of usable methane gas. AOX that is present in high concentrations is also eliminated in an economical way. However, no nitrogen elimination takes place in the anaerobic biological treatment, so digested sludge that is drawn from the digester, or more specifically, the filtrate that occurs when it is dewatered, is heavily loaded with ammonia and inorganic nitrogen compounds.
The invention is based on the task of suggesting a method for the anaerobic treatment of highly concentrated problematic waste water and preferably for the simultaneous aerobic treatment of communal waste water, by means of which an especially effective and extensive degradation of all contamination components present in the waste water, AOX, COD and nitrogen in particular, can be achieved, and by means of which the construction investment cost, energy consumption, and need for chemicals can be substantially reduced when compared with previously known methods.
The problematic waste water to be treated with the method according to the invention can occur in widely varying amounts. This can lead to fluctuations in the composition and solids content of the anaerobic treatment zone (digestion zone) contents. This can have a detrimental effect on the digestion and degradation process in the anaerobic treatment zone, particularly if the content of this zone is too greatly diluted.
The invention is also based on the task of configuring a method of the indicated type in such a way that conditions are assured in the anaerobic treatment zone that are as consistent as possible, independently of the fluctuations in admission.
Briefly stated, the invention in one preferred form is a method for treating waste water which comprises subjecting the waste water in an anaerobic treatment zone to an anaerobic degradation of the dissolved contents to form digested sludge and digested gas. Additional sludge is formed in an aerobic treatment zone of a waste purification plant. The additional sludge is sent to the anaerobic treatment zone. The additional sludge is also mechanically dewatered before sending it into the anaerobic treatment zone. The degree of dewatering is controlled in such a way that the concentration of dry substance in the anaerobic treatment zone does not fall below a pre-determined minimum value for long periods of time. The degree of dewatering is a function of the amount of waste water sent to the anaerobic treatment zone. The digested sludge is dewatered to produce a filtrate, and the filtrate is sent to an aerobic treatment zone of the waste purification plant for nitrogen degradation.
In accordance with another embodiment of the invention, a wastewater purification plant for treating waste water comprises a first admission inlet and a second admission inlet. Waste water of various origins enter the inlets. At least one aerobic treatment zone communicates with the first admission inlet for receiving waste water therefrom to form sludge. At least one anaerobic treatment zone communicates with the second admission inlet to receive waste water therefrom. Apparatus is provided for removing sludge from the at least one aerobic treatment zone. A mechanical dewatering device dewaters the sludge. Apparatus is provided for sending the dewatered sludge into the anaerobic treatment zone. A controller controls the degree of dewatering of the mechanical dewatering device. The at least one aerobic treatment zone includes a high-capacity zone and a low-capacity zone, wherein the low-capacity zone is operated with less sludge loading than the high-capacity zone. Apparatus may also be provided for withdrawing digested sludge from the anaerobic treatment zone. A device for the mechanical dewatering of the digested sludge forms a filtrate. Apparatus is also provided for sending the filtrate to at least one aerobic zone that is charged with the waste water from the first admission inlet. A precipitation zone for the chemical precipitation of waste water contents may also be disposed between the second admission inlet and the anaerobic treatment zone.
The invention is based on the principle of directing excess aerobic sludge from an aerobic treatment zone to the anaerobic treatment zone (digestion reactor) provided for the treatment of the problematic waste water, and thereby to adjust or regulate the degree of thickening (i.e., the solids content) in such a way that a solids concentration that is optimal for the anaerobic biocoenosis is maintained in the anaerobic treatment zone. In particular, undesirable dilution must be avoided in the anaerobic zone (digestion reactor).
In an advantageous development of the invention, two treatment sections that run parallel to each other, one for the aerobic treatment of communal waste water and the other for the anaerobic treatment of problematic waste water, are linked to each other in such a way that the anaerobic treatment section accepts excess sludge from the aerobic treatment section, and the filtrate that occurs in the aerobic treatment section from the dewatering of the sludge is directed into the aerobic treatment section. The advantageous effects achieved with these measures are explained in more detail in the following description of embodiments of the invention.